Known photo-detectors comprising e.g. photo-diodes or photo-resistors as light sensitive elements usually have wavelength-specific characteristics which are determined by a respective wavelength-dependent photo-response. Particularly, photo-detectors may be implemented in a flux feedback path of light emitting diode arrangements for sensing light emitted by a light emitting device.
Flux feedback control is expected to be implemented in a wide range of color controlled multi-light emitting diode (LED) products in the future. In order to control emitted light, a photo-detector having e.g. a stable characteristic over a wide frequency and/or a temperature range is required. The flux feedback signal may be used to monitor the temperature and aging induced flux decrease of the LEDs. Thus, it is necessary to have a photo-detector having a photo-response being stable with respect to temperature and aging. However, spectral measurements show that most silicon photo-detectors have a photo-response which is highly temperature dependent in the blue and infrared regime, whereas acceptable temperature stability is obtained in the green and red regime, respectively.
FIG. 7 shows the temperature dependent spectral photo response of a silicon photo-diode normalized by the spectral response of the photo-diode at a reference temperature, here 25° C. In FIG. 7 the reference sign 701 depicts the temperature dependence at 25° C., the reference sign 703 depicts the temperature dependence at 40° C., the reference sign 705 depicts the temperature dependence at 60° C., the temperature dependence 707 depicts the temperature dependence at 80° C., the reference sign 709 depicts the temperature dependence at 100° C. and the reference sign 711 depicts a temperature dependence at 120° C.
As depicted in FIG. 7, the temperature induced sensitivity variation has a maximum decrease at 450 nm which roughly corresponds to a typical emission wavelength of royal blue LED-emitters which are often employed in multi-LED systems using royal blue LEDs or phosphor converted white LEDs which rely on the royal blue LEDs as pump LEDs. The WO 2007/007238 A1 discloses such a concept for converting light in LEDs.
In a multi-primary system which is color controlled via flux feedback, a calibration step at room temperature relates the outputted flux of color strings to the photo-response signal measured simultaneously during calibration. If the sensor's photo-response is temperature dependent then the calibration relation will not hold anymore at a temperature of interest, e.g. at 80° C., which is to be expected in operating lamps. In the example of FIG. 7, the ratio of the sensitivity for blue light with respect to the sensitivity for green/red light varies with temperature. Thus, the flux feedback system will be provided with a too low blue signal compared to the green and red signal at higher temperatures which will lead to a color point drift of the system with temperature. Correcting for these errors requires measuring the temperature of the sensor and using the information on its specific temperature dependent sensitivity characteristics.
FIG. 6 shows an absolute spectral sensitivity as a function of temperature for the photo detector considered in FIG. 7, the photo-detector a silicon photo-diode. As depicted in FIG. 6, the sensor's sensitivity is generally lower in the blue regime than in the green and red regime. This is important for practical systems where the sensor is connected to a transimpedance amplifier and to an AD-converter with a finite resolution as depicted in FIG. 5.
FIG. 5 shows a typical sensor-transimpedance-AD-converter signal chain with a photo-diode 501, a feedback resistor 503, an operational amplifier 505 and an AD-converter 507.
The AD converter 507 outputs a linear signal with a given bit resolution for a certain voltage input range and the feedback resistor is chosen such that the resulting maximum signal fits within the certain range to obtain the best possible resolution of the signal. Due to the sensitivity of the sensor which is much lower in the blue regime, the maximum signal will be achieved for the signal when the red color string is on and the feedback resistor is chosen to output a suitable input signal to the AD converter. As the signal will be much lower for the case where the blue string is on, the resolution for the blue signal will be poor and this can lead to color control inaccuracies in the flux feedback system. One can introduce switchable feedback resistors while sensing the different color channels to adopt the input signal to the AD-converter to a good value for every color string but this adds considerably to the system complexity and costs. A further problem is that it is difficult to predict the behaviour of a particular photodiode. The above-addressed switching requires a precise knowledge on how the photodiode will behave at the operating temperature. If this behaviour is known then the photodiode can readily be calibrated.
Therefore, a sensor with more homogenous sensitivity in the visible spectral regime would contribute to a more reliable and a simple flux sensing of all color strings.